aw_clk_nm: fix incorrect use of abs()

abs() takes a (signed) int as input.
Instead, it was used with unsigned 64-bit integers.

So, add and use a new helper function to calculate a difference between
two uint64_t-s.

Reviewed by: manu
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D26307